LECTURE XV.] HISTORY OF CHEMISTRY. 305 



proof of decomposition, were only overcome some years after- 

 wards by Pebal, 38 who based his experiments on the state- 

 ment, first made by Bunsen, 39 that it was only possible to 

 distinguish mixtures of gases from homogeneous gases by 

 physical methods (diffusion or absorption). On causing the 

 mixture of gases obtained by heating ammonium chloride to 

 diffuse through an asbestos plug, Pebal was able to show, by 

 the colours imparted to litmus, that the gas in one part of the 

 apparatus possesses an alkaline and in another part an acid 

 reaction. 



In a similar manner, by means of diffusion, Wanklyn and 

 Robinson 40 endeavoured to show the breaking up of sulphuric 

 acid into sulphuric anhydride and water, and of phosphorus 

 pentachloride into phosphorus trichloride and chlorine. 



Deville attacked the conclusions which these chemists drew 

 from their experiments. 41 According to him, complete decom- 

 position was not necessary in order to accomplish a separation 

 of the constituents by means of diffusion, a dissociation involv- 

 ing a slight increase of pressure being quite sufficient. As the 

 products of decomposition are carried forward, further quan- 

 tities are formed, so that, given a sufficiently long duration of 

 the experiment, a complete separation of the constituents is 

 attained at a temperature which only corresponds to a very 

 slight decomposition. Deville points out that the vapour den- 

 sity of water is still normal at 1000, while at this temperature 

 it can be shown by diffusion that dissociation has already 

 taken place ; 42 and hence he considers that the abnormal 

 vapour density must be ascribed to the undecomposed vapour 

 of ammonium chloride. He finds what he regards as a positive 

 proof of this, in the considerable rise of temperature which he 

 believes he can recognise upon the intermixture of ammonia 

 and hydrochloric acid gases in a vessel previously heated to 

 350. 43 Robinson and Wanklyn having raised the objection 



a8 Annalen. 123, 199. 39 Bunsen, Gasometrische Methoden, 1857, 242. 

 40 Comptes Rendus. 56, 547. 41 Ibid. 56, 729. 4a Deville, Le$ons sur 

 la dissociation, 365. 43 Comptes Rendus. 56, 729. 



V 



